The present invention relates to an apparatus for the noninvasive measurement of the rate of blood flow according to ultrasonic Doppler effect techniques, the apparatus being of the type which is composed of ultrasonic transmitters and receivers for ultrasonic waves reflected by the flowing blood as well as a device for determining and evaluating the Doppler frequency shift between transmitted and received frequencies.
Such devices are known, one such device being disclosed, for example, in German Offenlegungsschrift [Laid-open Application] No. 2,461,264. These known devices have certain drawbacks, one of which is that they are intended for mesauring the rate of blood flow in relatively large blood vessels. Such devices operate in a frequency range of 5 to 10 MHz since at higher frequencies the attenuation of the ultrasonic waves in body tissue becomes so high that relatively deeply embedded blood vessels can not be monitored. Due to the frequencies employed, it is possible to measure the flow rate in large vessels, where it is of the order of magnitude cm/s, but not flow rates of the order of magnitude of mm/s, or microcirculation, which occur in the smaller blood vessels. Moreover, known devices do not permit differentiation between large and small vessels.